TRACKING THE GEOCHEMICAL EVOLUTION OF ARC MELTS FROM LOWER-CRUSTAL MASH ZONE TO MID-CRUSTAL MELT RESERVOIR, FIORDLAND, NEW ZEALAND

Diversification of mantle-derived melts is commonly thought to occur in lower crustal ‘MASH’ (melting, assimilation, storage, homogenization) zones at the crust-mantle boundary where basaltic melts undergo chemical interaction with host rocks. Understanding the mechanisms through which the chemistry of these magmas change from the lower crustal MASH zones to mid-crustal reservoirs is difficult because there are few places worldwide where exposures of lower and middle crustal melt reservoirs are preserved. Here, we focus on two plutons (Misty and Puteketeke Plutons) from a tilted-arc section in the Median Batholith (Fiordland, New Zealand), which preserves a lower- and mid-crustal reservoir, respectively. These coeval plutons present exposures of varying depths of the same magmatic arc, but with vastly different mineral assemblages and chemical compositions. The cause of the compositional disparities raises the question of how melts evolve as they ascend from the lower to middle crust. The Misty Pluton was emplaced from 122-115 Ma at 30-40 km depths and consists predominantly of two pyroxene diorites and hornblende diorites. The Puteketeke Pluton was emplaced at 20 km depth at ca. 122-120 Ma, and consists of biotite ± hornblende granites, granodiorites, and tonalites. Both plutons are classified as magnesian, and calc-alkalic to alkali calcic; however, the Misty Pluton is predominantly metaluminous whereas the Puteketeke Pluton is mostly peraluminous. Puteketeke Pluton samples are also much more silicic (>60 SiO₂ wt%) versus the Misty (<60 wt.%). The range of bulk-rock K₂O values for the Misty Pluton (0.3-3.0 wt%) overlap the slightly higher Puteketeke Pluton values (1.2-3.9 wt%), and Zr values also overlap (0-331.8 ppm for the Misty, and 72-175 ppm for the Puteketeke Pluton). In general, bulk-rock Y values are higher for the Misty Pluton (0.5-35 ppm) compared to the Puteketeke Pluton (2-13 ppm). Zircon δ¹⁸O values from the Misty (4.6-6.2‰; n= 85) overlap data from the Puteketeke (4.6-5.2‰; n=37) however, the latter displays a narrower range that is skewed towards lower values. These isotope data and differences in whole-rock geochemistry are consistent with interaction with mixing and/or partial melting of a low del¹⁸O source during transfer from the lower to middle crust.